If the angle of incidence differs from zero, spherical concave mirrors exhibit an astigmatism that increases with the angle of incidence. Although this can be corrected by using toroidal or elliptical mirrors, such mirrors are complicated to manufacture and hence expensive. Spherical concave gratings exhibit this astigmatism as well, because their image forming properties are inherently similar to those of spherical concave mirrors. It is true that with holographically produced concave gratings, good possibilities are known for correcting astigmatism by suitably selecting the source points (see, for example, R. Bittner, Optik, Volume 64, page 185, 1983); however, because of their blaze angle, conventional gratings are capable of greater efficiency in the visible and infrared range of the spectrum, so that even today they are still highly important. Furthermore, with holographically produced concave gratings, the fixation of the source points for correcting image aberrations imposes great restrictions on the geometric arrangement of the grating, its entrance slit and its spectral plane, so that a greater degree of freedom in terms of correction possibilities would be advantageous.